When using discrete light sources, for example light emitting diodes, to create an illumination effect, there is the need for the blending of the illumination created by these discrete light sources into a uniform lighting condition. For example, by fabricating a linear array of discrete light sources having a spacing therebetween, the light emitted thereby would typically be capable of creating neither a uniformly luminous linear line of light nor an even wash of light for wall washing or cove lighting, for example. In particular, problems arise when the illumination is being created by discrete light sources having a variety of colours therefore the blending of the separate colours is important for a smooth lighting condition.
Linear arrays of light emitting diodes mounted on circuit boards and enclosed in plastic tubes, such as TIR Systems Ltd.'s LightMark™ products, are used mainly for corporate identity applications in substitute of neon lighting. The disadvantage of these products is that the light emitting diodes constitute point sources of light and in order to ensure an even luminance distribution along the length of the tube, this tube must incorporate some means of diffusing the light.
Injection-molded and extruded plastics are the preferred material for diffusion. However, as discussed in IESNA Lighting Handbook, 9th Edition, the transmittance of these plastics varies between 30 and 70 percent for “white” (as opposed to clear) plastic. Thus, typically between 30 and 70 percent of the light emitted by the light emitting diodes is absorbed by this plastic tubing.
A second disadvantage of this type of diffuser, is that the plastic tubing must have sufficient cross-sectional area such that the distance between the light emitting diodes and the inner wall of the tubing is sufficient to enable even distribution of light and hence an even luminance distribution along the length of the tube. Failure to observe this requirement can result in an uneven luminance distribution and the perception of illumination hot spots. A consequence of this requirement is that the minimum tube diameter 5 is typically about two to three times the light emitting diode spacing, D 10 as illustrated in FIG. 1. In addition to this requirement, the printed circuit board 20 is preferably painted white or the same color as the light emitting diode 30 emission in order to facilitate diffusion of light that is reflected from the interior wall of the plastic tube 40. This combination of limitations and requirements, in addition to economic constraints on the number of light emitting diodes per unit length, which directly affects the spacing, D 10, practically limits the diameter of the plastic tubing to a minimum of approximately 20 mm. A minimum diameter of 20 mm can be acceptable for outdoor applications, however this configuration can be visually intrusive for many indoor applications, in particular for retail displays and shop window delineation. Thus this method of creating an even linear illumination does not achieve a sufficiently thin line of light for a number of applications.
Light guides made from a high refractive index material have been successfully employed to create a line of light from a point source. For example, U.S. Pat. No. 5,590,945 discloses an optical element that uses a total-internal reflection light guide to create a line of light from one or two light emitting diode point sources by internally reflecting the light along an axis, wherein beams of light escape the light pipe along the axis of the pipe. This form of lighting apparatus is designed such that the light guide is to be hidden inside a wall or panel. In addition, the length of the light line created is limited by the constraints on the length of the mold used to create the light guides. In addition, U.S. Pat. Nos. 5,165,772, 5,295,047 and 5,835,661 are also examples of the use of total internal reflection to create a line of light from a point source. While these methods and apparatus achieve a sufficiently thin line of light, the length of the line is effectively limited and the light guides cannot be easily configured end-to-end to create a longer continuous line of light. Furthermore, this method uses a very limited number of light sources, which in turn restricts the luminance and perceived visual brightness of the resulting line. As each of the above uses light guides to direct a point source of light into a line of light, and hence all suffer from the limitation on luminance. As such a line of light with high luminance and sufficient length cannot be achieved by using the above. An additional draw back is the fact that only a single pattern is achievable with this type of display.
It is known from prior art first introduced by Caulfield, H. J., Kinoform Diffusers, SPIE Vol. 25, Developments in Holography, 1971, that a holographic diffuser 50 can preferentially diffuse incident light 60 in a single direction, with commercially achievable distributions of 90 degrees or more by ¼ degree or smaller increments as illustrated in FIG. 2. International Patent Application No. PCT/US01/22311 discloses a method of using a variant of a holographic diffuser 70 called a “kinoform diffuser” to diffuse light emitted by linear fluorescent lamps 80 in order to minimize the problem of “lamp shadowing” between lamps aligned end to end as illustrated in FIG. 3.
Other issues arise when discrete light sources are used to illuminate a region, especially when discrete light sources are of varying colours enabling the adjustment of the cumulative illumination wavelength which is a blend of these individual colours. For example, the recent introduction of light emitting diodes and high brightness light emitting diodes to the gamut of commercially available light sources has made available such additional effects as colour changing or blending not achievable with fluorescent bulbs. For example, Crescent Lighting Limited Colourline products and Color Kinetics Ltd. iColor Cove® series utilise a linear array of red, green and blue light emitting diodes that are controlled such that any colour light may be projected. Using light emitting diodes is energy efficient, provides lamp longevity and provides durability. However, the fact that the individual light emitting diodes constitute different coloured point sources of light poses problems that require optical engineering solutions.
For example, one disadvantage of an array of point light sources is that the illuminated portion of a surface closest to the fixture will not be completely illuminated by the full light emitting diode array 90, thereby resulting in a scalloping effect 100 of the emitted light 110 as illustrated in FIG. 4. The optics associated with this type of lighting fixture, in particular with the light sources themselves, are designed for maximum ‘throw’, which is the maximum distance of the illuminated area from the fixture. As such, the quality of illumination on the portion of the wall closest to the fixture is compromised in the interest of throw.
A further disadvantage of the array of light emitting devices 115 is that any imperfections 120 on the illuminated surface, for example a raised mark on a wall, may be highlighted by multi-colored shadows 130 as illustrated in FIG. 5. Furthermore, any obstruction 140 positioned on the illuminated wall will cast a shadow and the point-source light emitting diode array 150 can result in multi-colored shadows or regions 160, 162, 164, 166, for example as illustrated in FIG. 6.
As such there is a need for a new system and method for the diffusion of illumination from discrete light sources in order to counter act the above inadequacies currently encountered.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.